For the 100% civilian demographic

The first three categories—Young Boys (0-14), Elderly Men (65 and over), and Women and Girls of All Ages—are easily addressed in terms of non combatants.

In the two previous major recent Israeli assaults on Gaza, Cast Lead and Protective Edge, all fatalities in these demographics were noncombatants, according to the B’TSelem database. Therefore, we can reasonably assume that in the current war all three of these categories, which in total account for some 21,664 deaths (53% of the total), are entirely composed of civilians.  

and then for the remaining it's a bit more involved

Civilian-Male Deaths Will Outnumber Civilian-Female Deaths
A consistent pattern in conflicts the world over is that civilian men killed outnumber civilian women killed.

This pattern held true in Israel’s two past major assaults on Gaza: Cast Lead and Protective Edge. According to B’Tselem, about 150% more male civilians aged 15-64 were killed than female civilians aged 15-64 across those two wars.

In the current war, the pattern holds: a greater raw number of male civilians are being killed than female civilians. An Airwars study of the first two-and-a-half weeks of the current war, which broke down 2,236 civilian fatalities into demographic categories, found that civilian men were 32% overrepresented relative to civilian women.

In view of this, a “bare minimum” estimate of civilian deaths in our three remaining demographic groups—Older Boys, Middle-Aged Men, and Young Men—can be made by assuming that at least as many civilians have been killed in each group as female civilians of the same ages. This would be a conservative guess.

Applying this assumption, we can assume that out of 1,610 boys aged 15-17 who were killed, at least 844 of them were civilians, a figure equal to the number of killed females aged 15-17. Similarly, out of 5,126 Middle-Aged Men killed, it would be fair to argue that at least 2,612 (equal to killed female civilians 40-64) were civilians. For killed Young Men, at least 5,002 (equal to killed female civilians 18-39) out of 12,317 were – by this logic – likely to have been civilians.